1. Field of the Invention
The present invention relates to the field of drug delivery systems, particularly those which involve microcapsules or microspheres. In addition, the present invention proposes methods for the preparation of particular multi-phase microspheres, as well as methods for their use in treating animals, including humans. The presently described microspheres also relate generally to the field of biodegradable in vivo delivery devices for proteins, peptides and other molecular compounds.
2. Background of the Art
The preparation and synthesis of a number of recombinant proteins has been the subject of a recent upsurge in research efforts. However, many proteins and peptides have short biological half lives, and as a consequence, delivery of most therapeutically valuable pharmaceuticals requires a regimen of multiple individual injections for desired therapeutic efficacy in a patient.
one way in which multiple individual injection regimens have been avoided is through the use of biodegradable microcapsules, preferably those which are capable of releasing the particular biologically active molecule continuously and at a controlled rate. Most preferably and ideally, such a system would provide for a continuous and controlled rate of drug delivery over a therapeutically valuable period of time. Such methods for the delivery of therapeutically valuable substances, such as proteins, peptides and pharmacologically active molecules, as well as systems for their effective delivery in an animal, have become important areas of research. However, for so-me biologically active molecules, a therapeutically effective delivery period will require continuous and sustained delivery over weeks or even months. In this regard, encapsulated pharmaceuticals have achieved only limited success, with days/weeks being the limit of even the most protracted systems available.
Present delivery systems include biodegradable microspheres and/or microcapsules which include biodegradable polymers, such poly d,l-lactic acid (PLA) and copolymers of tactic acid and glycolic acid (PLGA). These particular polymers are currently the most widely used biodegradable polymers employed in sustained release devices. Copolymers of tactic acid and glycolic acid may be obtained by polycondensation of lactic acid or glycolic acid in the presence.sup.34 or in the absence.sup.35 of a catalyst or other activator. Microcapsules prepared from such materials may be administered intramuscularly or by other parenteral routes.
Some PLA and PLGA microspheres exist which contain water-soluble compounds such as quinidine sulfate.sup.1,2, luteinizing hormone releasing hormone (LHRH) agonist.sup.5,6, and less water-soluble drugs such as phenobarbitone.sup.3,4. Biodegradable microspheres have also been reported which contain such drugs as steroids.sup.11,13, anticancer agents.sup.12,14, cardiac drugs.sup.15, and peptides.sup.6,16,17, specifically prepared with the polymers polylactic acid and copolymers of lactic acid and glycolic acid. The biological response modifiers (BRMS) have also been described in association with particular microcapsules.
However, the water solubility of a number of biologically active molecular compounds has proven to be one of the most limiting factors in optimizing molecular compound loading efficiency in biodegradable microspheres and/or microcapsules, specifically, when a conventional oil/water emulsion system is used in the solvent evaporation process. In this regard, it has been observed that the loading efficiency of water-soluble drugs into, for example, PLA or PLGA-polymeric microspheres is relatively low when conventional oil/water systems are used in a solvent evaporation process.sup.1,31. This has been attributed to the observation that such drugs readily diffuse into the aqueous outer phase of the emulsion system.
Some of the inventors' prior work.sup.1 addresses the low efficiency of drug loading due to solubility problems by adjusting the pH of the aqueous phase in the microencapsulation procedure. Adjusting the pH of the aqueous phase was shown to maximize loading efficiency of the particular water-soluble molecule, quinidine sulfate, in PLA microspheres, using a conventional O/W emulsion system in the solvent evaporation process. One of the inventors had found that by increasing the pH of the water soluble molecular compound solution, the solubility of the water soluble molecular compound will increase in the solution. Thus, the water soluble molecular compound is prevented/inhibited from diffusing from the microcapsule into the external aqueous phase.
Anhydrous emulsion systems have also been described in the preparation of microcapsules. For example, a phenobarbitone microencapsulation system was developed by Jalil and Nixon (1989).sup.18 using poly(1-lactic acid). Acetonitrile was used as the solvent ("W"-phase) for the poly(1-lactic acid) and the drug, and light mineral oil was used as the continuous dispersion medium (O-phase). However, in these microspheres, drug particles were dispersed in direct contact with the polymer matrix (conventional "matrix-type" system).
Direct contact of drug particles with a polymer matrix has been observed to contribute to degradation of a protein,.sup.7 perhaps by monomer and diner residues present in the polymer (inventors' unpublished observations). Polymeric degradation would also result in such "matrix" systems upon incorporation of proteins or enzymes in such a system, as direct contact with the polymer is again not prevented.
Most of the microspheres described in the literature belong to the class of "matrix-type" drug delivery capsules, in which the "foreign" (i.e. drug) particles are dispersed homogeneously in direct contact with the polymer. These processes also frequently involve direct contact between the drug and a polymer solvent, such as acetonitrile or methylene chloride. In these systems, direct contact between the particular biologically active molecule and the polymer, the polymer solvent or with enzymes in the biological system promote degradation of the intended pharmaceutical. Specifically, previous workers have shown that the monomer and dimer residues in the polymer may degrade the protein, and other workers have shown that proteins.sup.7,32 and enzymes.sup.33 in direct contact with the polymer will result in polymeric degradation over time.
In order to avoid interactions of sensitive substances (i.e., protein/peptides) with particular polymers, such as PLA and PLGA, in a delivery system, multiple-walled microcapsular devices employing conventional multiple emulsion techniques were proposed. For example, emulsion techniques have been reported using polymers to form microspheres having a multiple walled structure. For example, albumin containing-biodegradable microspheres have been reported by Saha et al..sup.25 using an O/W/O multiple emulsion solvent evaporation technique. In addition, Morris et al..sup.24 described a three-ply walled microsphere composed of acacia/ethylcellulose/acacia layers prepared by using a W/O/W multiple emulsion solvent evaporation technique. However, drug loading capacities of these devices are limited in that sufficient amounts of the polymer must be present to facilitate formation of a stable microsphere. While the use of highly potent drugs may circumvent the drug-loading capacity problem, sufficiently "controlled" biodegradable systems for providing a constant and controlled drug release to justify the high cost of potent drug proteins and peptides are not currently available.
Despite the advantages to be gained from a multi-walled microsphere delivery device, the systems thus far described are limited to relatively low levels of drug loading capacity, since sufficient polymer must be present in order to form a sufficiently stable -microsphere. Carefully controlled drug release and high efficiency in drug loading (preferably more than 90%) are required features in clinically efficacious microencapsulation techniques, particularly in light of the high potency and in many cases, prohibitive costs of synthetic peptide/protein drugs.
Drug loading capacity loading efficiency, stability and controlled releases have been observed as limiting factors even when highly potent water soluble drugs are incorporated into microparticles. Thus, difficulties relating to the accomplishment of carefully controlled and sufficiently extended drug release rates remain. In addition, potential drug degradation from direct contact of the particular biologically active molecule and the polymer as well as from contact of the polymer with enzymes in vivo, potentially remain.
Present techniques used by biotechnology and pharmaceutical companies to encapsulate peptides in biodegradable polymers utilize a solvent-nonsolvent system which unfortunately suffers from the disadvantage of high solvent residuals, poor content uniformity of the peptide in the microspheres, and instability due to the contact of the biological agent with the polymer, organic solvent (e.g. methylene chloride, acetonitrile), and some cases, a surfactant.
Thus, while some biotechnology companies in the United States, Japan and Europe have achieved some measure of success in developing potent peptides and proteins for a variety of clinical conditions, major challenges remain to be solved by pharmaceutical scientists in developing stable and optimized control-release formulations for potent water soluble bioactive agents. Thus, a microcapsule delivery system which permitted the efficient loading of water-soluble biologically active molecules in a biodegradable carrier system would provide a medically significant advance in the clinically valuable and cost-effective preparations for long-term in vivo drug delivery of potent water-soluble chemicals, proteins and peptides.